Aggregates (set functions)

This section describes aggregates (also described as set functions in
ANSI SQL-92 and as column functions in some database literature). They
provide a means of evaluating an expression over a set of rows. Whereas the
other built-in functions operate on a single expression, aggregates operate
on a set of values and reduce them to a single scalar value. Built-in aggregates
can calculate the minimum, maximum, sum, count, and average of an expression
over a set of values as well as count rows.

An ORDER BY clause (using an alias
name) if the aggregate appears in the result of the relevant query block.
That is, an alias for an aggregate is permitted in an ORDER BY clause if
and only if the aggregate appears in a SelectItem in a SelectExpression.

All expressions in SelectItems in the SelectExpression must
be either aggregates or grouped columns (see GROUP BY clause).
(The same is true if there is a HAVING clause without a GROUP BY clause.)
This is because the ResultSet of a SelectExpression must
be either a scalar (single value) or a vector (multiple values), but not a
mixture of both. (Aggregates evaluate to a scalar value, and the reference
to a column can evaluate to a vector.) For example, the following query mixes
scalar and vector values and thus is not valid:

-- not valid
SELECT MIN(flying_time), flight_id
FROM Flights

Aggregates are not allowed on outer references (correlations).
This means that if a subquery contains an aggregate, that aggregate cannot
evaluate an expression that includes a reference to a column in the outer
query block. For example, the following query is not valid because SUM operates
on a column from the outer query:

SELECT c1
FROM t1
GROUP BY c1
HAVING c2 >
(SELECT t2.x
FROM t2
WHERE t2.y = SUM(t1.c3))

A cursor declared on a ResultSet that includes an aggregate
in the outer query block is not updatable.